A combined wire structure, made up of longer periodic Si microwires and short nanoneedles, was prepared to enhance light absorption using one-step plasma etching via lithographical patterning. The combined wire array exhibited light absorption of up to ~97.6% from 300 to 1100 nm without an anti-reflection coating. These combined wire arrays on a Si substrate were embedded into a transparent polymer. A large-scale wire-embedded soft film was then obtained by peeling the polymer-embedded wire portion from the substrate. Optically attractive features were present in these soft films, making them suitable for use in flexible silicon solar cell applications.
©2010 Optical Society of America
In recent years, various silicon nanowire structures have been investigated for the development of cost-effective, higher efficiency solar cells [1–14] because Si nanowires demonstrate strong, broadband optical absorption due to light trapping effects caused by multiple scatterings. Enhancement of the light trapping ability leads to improved solar cell efficiency via the increased collection of short-circuit photocurrents. The radial p-n junction  requires a shorter diffusion length for carrier collection and is capable of utilizing low-purity silicon since the required diffusion length, LD, is significantly shorter (LD ≤ wire radius) than that of a planar solar cell, while maintaining the light absorption enhanced by light trapping .
Several methods, such as vapor-liquid-solid [9–12], electroless etching , and deep reactive ion etching [14,15], have been attempted to prepare radial p-n junction silicon wires. In solid-state solar cells with a radial junction geometry, the conversion efficiencies of ~5.3% for Si nanowires , ~5.7% for microwires , and ~8.45% for co-integration of micro- and nanowires  are the highest values reported to date. Although the nanowires demonstrate strong light absorption, their nanoscale geometries do not allow for the formation of a radial junction structure with a surface passivation layer. Unlike nanowires, microwires can be used to create radial junction wire solar cells, but their optical absorptions are weaker than those of the nanowires due to higher reflection. To resolve this issue, Um et al.  suggested the use of a cost-efficient electroless etching technique in which high-efficiency Si wire solar cells possess not only strong light absorption due to nanowires, but also an efficient carrier collection induced by the radial p-n junction of the microwires.
However, the combined wire array prepared via the electroless etching method suffers from a relatively high dark current due to the increased nanowire length which is the same as the microwire length. In contrast to radial junction microwires, since the nanowires are responsible only for light absorption without generating collected charge carriers, the increase in nanowire length causes an increase in the total amount of surface recombination. Hence, the micro- and nanowire lengths must be independently controlled to produce nanowires much shorter than the microwires by optimizing the optical absorption.
To address this challenge, we proposed a unique plasma etching technique utilizing inductively coupled plasma for producing ideally combined wires (co-integration of long microwires with very short nanoneedles). For a large-scale (≥8 inch) wafer patterning process, plasma etching is usually considered to be the most suitable method because of its high process reproducibility with good wafer uniformity. One thing we noted is that the lengths of the microwires and nanoneedles were able to be controlled independently by adjusting the source gas ratio during plasma etching.
In addition, we attempted to transfer these combined wires into a polymer-embedded thin film for flexible Si wire-embedded thin film solar cell applications. To lower the overall production costs, the silicon substrate remaining after detaching the wire arrays should be reusable after chemical planarization. Compared to the long nanowires studied in the previous work , the shorter nanoneedles which formed between the long microwires made it easier to fill in the gaps between the microwires with a transparent polymer. The optical characteristics of these wire arrays were investigated to demonstrate their features for flexible wire-embedded thin film solar cell applications.
A periodic silicon microwire array on an 8-inch Si wafer was prepared using photolithography followed by plasma etching. A 1.5-µm-thick SiO2 layer was deposited onto a p-type Si(100) substrate (B, 3−25 Ωcm) using high-density plasma chemical vapor deposition, and then the substrate was patterned using reactive ion etching in a mixture of CF4 and CHF3 plasma. Periodic SiO2 dot arrays (2 µm in diameter) were used as etch masks. A plasma etching process, including etching (SF6) and polymerization (C4F8) steps, was adopted to define the high aspect ratio Si microwire arrays. The wire pitch (center-to-center distance between microwires) and diameter were determined according to the patterned size of the photo mask. The wire morphologies, Si microwires or combined wires, could be selected by adjusting the ratio between the etching and polymerization steps without using an additional lithographical step, as depicted in Fig. 1 . Typically, combined wires were fabricated with a source power of 2500−3000 W, a stage power of 100 W, and a gas pressure of 45 mTorr under a source gas SF6/C4F8 ratio of 400/200. After plasma etching, the resulting polymeric coating and etching damage on the wire sidewalls were removed using a sulfuric-peroxide mixture (H2SO4:H2O2 = 2:1) solution followed by thermal oxidation (1000°C for 116 min). The remaining oxide was stripped using a buffered oxide etchant. For flexible thin film applications, the fabricated wires were embedded into a polymer in which the polydimethylsiloxane (PDMS) casting was applied onto a Si wire array. After subsequent curing at 150°C for 3 hrs, the wire-embedded films were peeled from the substrate using a long, thin knife.
3. A periodic simple Si microwire array
The wire morphologies were characterized using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Figure 2 shows plan-view (inset) and cross-sectional SEM images of Si microwires for two different wire lengths and pitches. At the same wire diameter of ~2 μm, panel A is ~20 μm in length and ~10 μm in pitch, and panel B is ~50 μm in length and ~7 μm in pitch. Square-patterned Si microwires resulted in a packing fraction of ~3.1% for the 10 µm pitch and ~6.4% for the 7 µm pitch.
To quantitatively compare the optical properties, reflection measurements were performed over the 300−1100 nm wavelength range using a UV−Vis/NIR spectrophotometer (Lambda 750, Perkin Elmer) equipped with a 60 mm integrating sphere to account for the total (diffuse and specular) light reflected from the sample. Compared to the planar Si wafer, the optical reflection amounts of the Si microwires were all lower (see Fig. 3 ) because the microwires acted as a buffer layer, intervening a large difference in the effective refractive indexes between the air and substrate. Also note that the reflection generally decreased with increasing wire length and/or decreasing pitch, which agreed well with the increase in practical total surface area per unit projected sample area. In cases without any topological features such as wire arrays, the total surface area of the sample was equal to the unit projected sample area. The light path length increased with increasing wire length and/or decreasing pitch, which also enhanced the light trapping effect. Hu and Chen have previously reported this effect in detail in their simulation work .
4. A combined wire structure using periodic microwires co-integrated with small nanoneedles
To achieve greater light absorption, the optical characteristics of the Si microwires were modified by controlling the geometrical characteristics of the wire array such as length and pitch. To further enhance the absorption capability, small 5-µm-long nanoneedles were purposely co-integrated between the long, periodic microwires. This approach to forming combined wires was realized by increasing the source gas ratio of the SF6/C4F8. Figures 4(a-c) show the three different combined wire samples which used the same nanoneedles but different microwire lengths. Figure 4(d) compares the wafer-scale uniformity of the patterned wire arrays, i.e., Si microwires (right) and combined wires (left). In contrast to the mirror-like features of the polished bare wafer, the dark black color revealed remarkable suppression of reflection over the visible wavelength range. In particular, note that the combined wires showed much darker features, implying stronger reflection suppression, compared to the slightly shiny nature of the Si microwires.
The total reflection spectra of these three combined wires are shown in Fig. 5(a) . Compared to the Si microwires, the overall reflection of the combined wires was significantly suppressed from 35% to 5% due to the graded refractive index (GRI) effect of the nanoneedles. As we previously reported , GRI behavior at the transition layer of refractive indexes can greatly reduce unwanted reflection. In this work, Si nanoneedles reduced the large mismatch in refractive indexes between the microwires and substrate. The light absorption of these combined wire samples can be calculated using A(%) = 100 − R(%) − T(%), where A, R, and T are the absorption, reflection, and transmission, respectively. The absorptions of the combined wire samples are plotted in Fig. 5(a), illustrating that more than 95% of the incident visible light was absorbed.
In particular, the long wire sample (70 µm in length) demonstrated a maximum absorption of 97.6%, which was beneficial for obtaining a high-efficiency solar cell. The light absorbing capability was also enhanced with increasing lengths of microwires, consistent with the results for the simple Si microwires, since the trapped light will travel further in the microwires, greatly increasing the probability of absorption. Figure 5(b) compares the specular reflections in both the combined wires and Si microwires for the same wire length (50 µm). Specular reflection in the combined wires was greatly suppressed because the nanoneedles effectively scattered the incident light to produce diffuse reflection. This result clarifies the effectiveness of the nanoneedles for enhancing light absorption.
5. Flexible thin film photovoltaic applications
5.1 Fabrication and morphologies of wire-embedded PDMS films
For large scale (≥10 cm2) flexible thin film applications, the fabricated Si microwires and combined wires were embedded into a PDMS polymer. After curing at 150°C for 3 hrs, the wire-embedded films were then detached from the substrate. The cross-sectional SEM image shown in Fig. 6(a) shows the Si microwire-embedded film (SWF) just separated from the substrate and having a wire pitch and length of 7 and 50 µm, respectively. Note that the wire bottoms protruded normally from the PDMS and that the PDMS profile was concave at the separated bottom face [see the inset in Fig. 6(a)]. The concave profile originated from the capillary effect of the liquid polymer as it was infiltrated into the wire arrays. The remnant air moved into the wire bottoms during infiltration and caused the formation of equilibrium menisci of the liquid polymer close to the wire bottoms. The equilibrium radius of curvature at the concave menisci was dependent upon the sidewall angle and morphology of the wire bottoms which were controlled using the etching recipes. This corrugated bottom structure made it easier to form the periodically defined, nanostructured metal film via simple metal deposition.
The combined wire-embedded film coated with an Al back reflector (CWAF) is shown in Fig. 6(b), in which the nanoneedles are clearly revealed between the microwires [see the inset in Fig. 6(b)]. The flexibilities of these wire-embedded films were observed to be superior, i.e., the detached films could be bent or rolled without any remarkable damage or dislodging of the embedded Si wires from the PDMS [see Figs. 6(c) and 6(d)]. These composite films, which consisted of single-crystalline Si microwires/nanoneedles integrated with flexible, chemically stable PDMS , suggest an attractive photovoltaic solution for photostable energy conversion (via crystalline silicon) in conjunction with mechanical robustness and reliable air passivation (via flexible PDMS).
5.2 Principles of optical confinement in wire-embedded PDMS films
Figure 7 explains methods to increase light absorption in wire-embedded films. First, the presence of Si nanoneedles greatly decreases the transmission loss of incident light, while scattering and absorbing the transmitted light. In addition, the amount of reflection can be further decreased via the GRI effect, because the rather large mismatch in refractive indexes between microwires and air can be decreased near the separated interface due to the presence of tapered nanoneedles. Second, a thin metal film coated onto the back surface of the wire-embedded films can be used as a back reflector from which the transmitted light is easily reflected back into the wires to increase the effective path length of the light. Third, metal nanostructures formed on a back reflector can couple the incident light into the surface plasmon polariton (SPP) modes in which electromagnetic excitations coupled to electron oscillations propagate along the metal-dielectric interface, resulting in sub-wavelength optical confinement.
The total transmissions, reflections, and absorptions of these four types (see Fig. 7) of wire-embedded films without the influence of substrates were measured as shown in Figs. 8(a–c) , respectively. The SWF sample exhibited an average absorption of ~14% in a spectral range of 300−1100 nm. Wide spatial gaps between the microwires, composed of PDMS, caused a high transmission loss of incident light, as depicted in Fig. 7(a). The SWF sample exhibited an average transmission of 75−87% for λ = 300−1100 nm. The addition of Si nanoneedles between the microwires was found to be effective for not only suppressing this large amount of transmission loss, but also for reducing the reflection loss via the GRI effect, as noted earlier.
Effect of Si nanoneedles. In Fig. 8(c), the combined wire-embedded film (CWF) structure exhibits an average absorption of ~53%, greater than that of the SWF because of the presence of Si nanoneedles. These nanoneedles could increase the total absorption by 3- to 9-fold in a spectral range of 300−1100 nm due to the GRI effect [see Fig. 8(d)]. These results clearly demonstrated the effectiveness of the nanoneedles for enhancing light absorption in wire-embedded films. Their insufficient optical thicknesses , however, greatly decreased the absorption capability in the near-infrared (NIR) region, i.e., light absorption was ~5% for λ = 1100 nm but ~64% for λ = 300 nm. For broad band absorption, an additional possible approach is to deposit an Al layer onto the backsides of the SWF and CWF.
Metal thin film as a back reflector. The Si microwire-embedded films coated with an Al back reflector (SWAF) were formed by depositing a 300-nm-thick Al thin film onto the back side of a SWF sample, as shown in Fig. 7(c). Applying a metal film to the backside, in general, enhances light absorption due to the reflector effect, thus increasing the path length. The absorption efficiency of the SWAF was increased greatly compared that of a SWF, as shown in Fig. 8(c). However, since the reflection of the SWAF was also found to be higher than that of a SWF, the light reflected by the metal in the SWAF was not absorbed fully into the Si microwires [see Fig. 8(b)].
Compared to a simple microwire array, the CWAF exhibited a higher average absorption of ~92% in the spectral range from 300−1100 nm. This remarkable increase [see Fig. 8(d)] in broad band light absorption can be understood based upon the two principles of back reflection effects [20,21] and plasmonics [21–23]. Incident light transmitted through the Si microwires and nanoneedles (27−61% for 300−1100 nm) is reflected back into the Si wires via the thin Al reflector. As a result, the incident light will traverse further with the increased effective path length of light throughout the Si wires. Also note that the CWAF exhibited an absorption enhancement of up to ~80 near the NIR range [see the inset in Fig. 8(d)]. In the wire arrays, perfect random scattering combined with a lossless back reflector is known to theoretically enhance the effective path length by a factor of 4n 2 , corresponding to ~50 of that in Si. This implies that the tremendous increase (~80) in absorption enhancement observed near the NIR range requires an explanation in addition to the back reflector effect.
Plasmonic effects by metal nanostructures. Near the NIR region shown in Fig. 8(b), the reflection of the CWF was observed to be higher than that of the CWAF, which implies that the CWAF sample absorbed more incident light than did the CWF. This phenomenon can be understood based on the effect of SPPs at the metal-dielectric interface. As shown in Fig. 6, the many protruding wire-bottoms with concave menisci at the back side of the CWF naturally caused the formation of periodically-defined, nanostructured Al gratings at the interface between the CWF and Al film during conversion to the CWAF sample. These Al gratings might be an effective aid in overcoming the momentum mismatch between the incident photons and the SPPs to transform the incident light into an SPP wave. In fact, excitations of SPPs on continuous metal films with nanohole arrays, similar to our structure, have been shown  to propagate along the surface of the samples.
As shown in Fig. 8(b), however, the CWAF sample still exhibited a reflection of 8−17% for the long wavelength range from 900−1100 nm, even though the incident light was trapped in the flexible thin film due to the generation of SPPs. This reflection loss may have originated from the scattering effect of SPPs caused by discontinuities in the geometry  or from dielectric permittivity ; in our work, the energy in the SPP mode was partially scattered into the optical mode when geometrical discontinuities, such as the protruding Si wire bottoms alternating with PDMS, interacted with the propagating SPP waves. For a single scattering, 10−30% of the SPP energy was theoretically [26,27] estimated to be scattered into the optical modes. Some portions of scattered light that could not be absorbed into the wire-embedded film were considered to be reflection loss in the NIR. In addition, with the absorption enhancement of the metal back reflector, and taking this plasmonic effect into account, the absorption enhancement of ~80 observed in the CWAF sample is reasonable.
We fabricated various combined wire structures made up of long periodic Si microwires and short nanoneedles using photolithography and plasma etching, in which a high optical absorption efficiency was observed without the need for an anti-reflection coating. We also fabricated a combined wire-embedded film coated with an Al back reflector (CWAF) in which the combined wires were embedded within a flexible and transparent PDMS polymer. These CWAF films could be bent or rolled without damage or dislodging of the embedded Si wires. Because of the plasmonic effect and due to the nanostructured Al gratings, the Al back-reflected CWAF exhibited an average absorption of ~91.5% for the entire spectral range of 300−1100 nm, along with a remarkable enhancement (~80) in NIR absorption, presenting it as a possible platform for next-generation flexible thin film solar cells.
This work was supported by the Pioneer Research Center Program (No. 2010-0002231), Mid-Career Researcher Program (No. 2010-0000231), and Nano R&D Program (No. 20090083229) through the National Research Foundation (NRF) grant funded by the Ministry of Education, Science and Technology (MEST). This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 2009T100100614). In addition, we are grateful to the National Nanofab Center for technical assistance. K.-T. Park and H.-D. Um acknowledge the financial support of the fifth-stage Brain Korea 21 Project in 2010.
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